1. Field of the Invention
The present invention relates to a toner for use in developing an electrostatic image. In addition, the present invention also relates to a developer including the toner. Further, the present invention relates to an image forming method and an image forming apparatus, which form images using the toner.
2. Discussion of the Background
In conventional electrophotographic image forming apparatus and electrostatic recording apparatus, electrostatic latent images or magnetic latent images are visualized using toner. Specifically, electrophotographic image forming methods typically include the following processes:
(1) forming an electrostatic image on an image bearing member such as photoreceptors (electrostatic image forming process);
(2) developing the electrostatic image with toner to prepare a toner image on the image bearing member (developing process);
(3) transferring the toner image onto a receiving material such as paper sheets (transferring process); and
(4) fixing the toner image on the receiving material upon application of heat or the like thereto, resulting in formation of a recorded image (fixing process).
The toner used for developing an electrostatic image is a particulate colored material in which a colorant, a charge controlling agent, etc., are included in a binder resin. The toner preparation method is broadly classified into pulverization methods and suspension polymerization methods.
The pulverization methods typically include the following processes:
(1) Dispersing a colorant, a charge controlling agent, an offset preventing agent, etc., in a thermoplastic resin by a melt blending method;
(2) Cooling the melted toner constituent mixture to solidify the mixture; and
(3) Pulverizing the toner constituent mixture, followed by classifying, resulting in preparation of a mother toner (toner particles).
By using such pulverization methods, toner having reasonable properties can be obtained, but only limited materials can be used therefor (i.e., the flexibility of the methods in material selection is low). Specifically, the toner constituent mixture prepared by a melt blending method has to be pulverized and classified by a low-cost pulverizer and classifier. Namely, the toner constituent mixture has to be brittle enough to be pulverized. When pulverizing such a brittle toner constituent mixture, the resultant particles (i.e., the toner particles) have a relatively wide particle diameter distribution. In order to produce toner images having a good combination of resolution and half tone property using such toner particles, it is necessary to remove fine particles having a particle diameter of not greater than 5 μm and coarse particles having a particle diameter of not less than 20 μm from the toner particles, resulting in deterioration of yield of the toner. In addition, it is difficult for such pulverization methods to uniformly disperse a colorant, a charge controlling agent, etc., in a binder resin, and thereby a problem in that one or more of the properties (such as fluidity, developability, durability and image qualities) of the resultant toner deteriorate tends to be caused.
Further, the toner prepared by such pulverization methods is typically used for heat fixing methods in which a toner image on a receiving material is fused by a heat roller to be fixed thereon. In this case, when the temperature of the heat roller is too high, the toner is excessively melted, a hot offset phenomenon in that the melted toner is adhered to the heat roller is caused. In contrast, when the temperature of the heat roller is too low, the toner is not sufficiently melted, thereby causing a poor fixing problem in that the toner image is not sufficiently fixed.
Recently, a need exists for an energy saving image forming apparatus (such as copiers) having a small size. Therefore, the toner used for such an image forming apparatus preferably has a high hot offset temperature and a low fixable temperature (i.e., a good combination of hot offset resistance and low temperature fixability). In particular, in full color copiers and printers, the glossiness of fixed toner images and the color tones of fixed combined color toner images are important image qualities. Therefore, toner having a lower melting point is desired for such full color copiers and printers.
However, toner having a low melting point typically has not only a low hot offset temperature but also a poor preservability under high temperature and high humidity conditions. In order to prevent occurrence of the hot offset problem, a technique in that a release agent such as silicone oils is applied to a heat fixing roller to impart good releasability to the heat fixing roller is used for such full color image forming apparatus. In order to use such a technique, the image forming apparatus has to include an oil tank and an oil applicator, resulting in complexity and jumboization of the image forming apparatus. In addition, the heat roller tends to easily degrade due to the applied oil, and therefore the image forming apparatus requires periodical maintenance. Further, the image forming apparatus tends to cause a problem in that the oil applied to the heat fixing roller is adhered to a copy sheet, thereby causing a problem in that the color tone of the images is changed.
In order to prevent occurrence of the hot offset problem without applying an oil to a heat fixing roller, a technique in that a release agent such as waxes is included in the toner is typically used. In this regard, the releasability of the toner largely depends on the dispersion condition of the wax in the toner. Specifically, when the wax is dissolved in a binder resin, the toner cannot exert good releasability. When the wax is present as particles (domains) in a binder resin, the toner can exert good releasability. In this regard, when the particle diameter of the wax particles in a binder resin is too large, the amount of the wax present in the surface portion of the toner particles is relatively large. In this case, the toner particles tend to be agglomerated, and thereby the fluidity of the toner is deteriorated. In addition, a filming problem in that the wax is adhered to the carrier of the developer and the image bearing member such as photoreceptors, resulting in formation of a film of the toner thereon, thereby deteriorating the qualities of recorded images tends to be caused. In contrast, when the particle diameter of the wax particles in a binder resin is too small (i.e., the wax is excessively dispersed finely in a binder resin), the toner cannot exert good releasability.
It is difficult for the pulverization methods to control the particle diameter distribution of the wax dispersed in the toner particles. In addition, when the toner constituent mixture is pulverized, pulverization is typically caused at the wax domains. Therefore, the wax is typically present at the surface of the toner particles. Accordingly, the fluidity deterioration problem and the filming problem mentioned above tend to be caused.
In attempting to solve the problems, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 09-319144 and 2002-284881 (corresponding to U.S. Pat. No. 7,005,480) have disclosed resin solution suspension methods including the following processes:
(1) Dissolving a resin, which has been synthesized by a polymerization method, in a solvent to prepare a resin solution;
(2) Dispersing the resin solution in an aqueous medium in the presence of a dispersant such as surfactants and water-soluble resins, and a dispersion stabilizer such as particulate inorganic materials and particulate resins to prepare a dispersion; and
(3) Removing the solvent from the dispersion by heating, depressurizing or the like method to prepare toner particles.
By using these methods, toner having a relatively sharp particle diameter distribution can be obtained without performing a classification operation.
In general, toner includes a binder resin in an amount of not less than 70% by weight, wherein the binder resin is obtained from oil resources. However, recently there are fears of deletion of oil resources and global warming due to discharge of a large amount of carbon dioxide caused by using a large amount of oil sources. In attempting to solve the problems, techniques in that resins obtained from plants, which grow by absorbing carbon dioxide, are used as binder resins have been proposed. In this case, carbon dioxide is circulated in the environment. Therefore, the techniques may prevent occurrence of the problems.
For example, JP-A 07-120975 proposes to use polylactic acid as a binder resin. However, polylactic acid exerts less thermoplastic action than polyester resins in the fixing process because of including ester bonds in a relatively high content. In addition, the toner constituent mixture has very high hardness, and thereby a problem in that the toner constituent mixture cannot be easily pulverized, resulting in deterioration of productivity tends to be caused.
JP-A 09-274335 proposes a toner including a colorant, and a polyester resin prepared by subjecting a composition including lactic acid and a tri- or more-functional oxycarboxylic acid to a dehydration polycondensation reaction. Since a polyester resin is prepared by subjecting the hydroxyl group of lactic acid and the carboxyl group of the oxycarboxylic acid to a dehydration polycondensation reaction in this proposal, the resultant polyester resin has a large molecular weight. Therefore, the resultant toner has poor sharp melting property, thereby deteriorating the low temperature fixability of the toner.
JP-A 2001-166537 discloses a toner including a polylactic acid based biodegradable resin and a terpene phenolic copolymer in attempting to improve the thermal properties of the toner. However, the toner cannot have a good combination of low temperature fixability and hot offset resistance.
Since these toners are prepared by pulverization methods, the toners have problems in that considerable losses are caused, and there are fears of environmental destruction due to disposal of the waste toner. In addition, the energy needed for such pulverization methods is relatively high. Therefore, it is necessary to reduce environmental burdens.
As JP-As 07-33861 and 59-96123 have disclosed, polylactic acid, which is a resin derived from plants and which is easily available, can be synthesized by subjecting lactic acid to dehydration condensation reaction or subjecting a cyclic lactide to a ring opening reaction. Therefore, the resin solution suspension methods mentioned above can be used for preparing a toner including polylactic acid. However, polylactic acid has a problem in that L-polylactic acid or D-polylactic acid itself has high crystallinity and has very poor solubility in organic solvents.
In addition, since polylactic acid has low crystallization speed, it is difficult to control the crystallization state of polylactic acid included in the toner prepared by a resin solution suspension method. In this regard, polylactic acid having an amorphous state has poor heat resistance, i.e., poor hot offset resistance. Further, there is a case where the toner prepared by a resin solution suspension method includes a mixture of polylactic acid having high crystallinity and polylactic acid having low crystallinity. When such a toner is agitated in a developing device, the toner particles tend to be broken at a portion consisting of polylactic acid having low crystallinity because such a portion has poor impact resistance. In this case, fine toner particles are formed, and thereby the charge amount of the toner is decreased with time, resulting in deterioration of image qualities (such as image density).
Because of these reasons, a need exists for a toner which has a good combination of fixability (low temperature fixability and hot offset resistance) and high temperature preservability and which can produce high quality images without causing the filming problem.